An ever increasing number of vehicles has resulted in increasing air pollution, particularly in urban areas. Therefore, a series of ever stricter emission standards have been imposed to mitigate these pollution problems.
The main pollutants of concern are hydrocarbons (HC), carbon monoxide (CO), oxides of nitrogen (NOx) and particulates (i.e. soot).
Gasoline fuelled spark ignition (SI) engines typically operate under an air lead mode, wherein a throttle valve controls the rate of air supplied to the engine in response to the torque requests of the driver and the fuel supply system supplies an amount of fuel based upon the air supply rate to obtain a desired air/fuel ratio.
In order to reduce the emissions of HC, CO and NOx from gasoline fuelled spark ignition (SI) engined vehicles, three way catalytic converters have been widely implemented, such three way catalytic devices incorporating a reduction catalyst, to reduce NOx to N2 and O2, and an oxidation catalyst, to oxidise CO to CO2 and HC to H2O and CO2. However, for correct operation of a three way catalyst, the vehicle engine must be controlled to operate stoichiometrically where the amount of oxygen supplied to the combustion chamber corresponds to that required for complete combustion of the amount of fuel supplied. For gasoline, this corresponds to an air/fuel ratio of 14.7 parts air to 1 part fuel.
When there is more air, and hence oxygen, than required, then the system is said to be running lean, and the system is in oxidizing condition. In that case, the converter's two oxidizing reactions (oxidation of CO and HC) are favoured, at the expense of the reducing reaction. When there is excessive fuel, then the engine is running rich. The reduction of NOx is favoured, at the expense of CO and HC oxidation. If an engine could be held at the strict stoichiometric point for the fuel used, it is theoretically possible to reach 100% conversion efficiencies.
Unlike spark ignition gasoline fuelled engines, diesel fuelled compression ignition engines are normally operated under an fuel lead mode wherein a fuel is injected into the combustion chambers (or into a pre combustion chamber) at a rate based upon the torque demand of the driver, and air is drawn into the engine to supply oxygen for the combustion of such fuel by the pumping action of the engine and possibly also by the boosting effort of a turbocharger/compressor. An advantage of this form of combustion is the low pumping losses, particularly at part load, because of the absence of a throttle valve in the intake. However, such fuel lead operation results in a great variation of the air/fuel ration of the combustion mixture during engine operation.
As discussed above, in a compression ignition engine, torque demands are met by varying the amount of fuel supplied to the engine and there is normally no precise control of the amount of air supplied (no throttling in the intake) and thus, while an oxidation catalyst can be used to oxidise CO to CO2 and HC to H2O and CO2, excess oxygen in the exhaust gases due to the lean burn conditions normally prevents reduction of NOx to N2 and O2 and thus prevents the use of a three way catalyst.
The big challenge for future diesel emissions is the more tightly control the air/fuel ratio to control the composition of the exhaust gases and to enable correct operation of various exhaust gas treatment devices located within the exhaust system.
For example, it is known to use after-treatment of the exhaust gases to remove the NOx.
Selective catalytic reduction (SCR) can be used to reduce the NOx,wherein a gaseous or liquid reductant (most commonly ammonia or urea) is added to the exhaust gas stream and is absorbed onto a catalyst. The reductant reacts with NOx in the exhaust gas to form H2O (water vapour) and N2 (nitrogen gas). However, SCR is very sensitive to fuel contaminants, requires a limited (high) temperature operation window and is costly.
Another method of exhaust after-treatment to remove the NOx is the use of a lean NOx trap. This requires rich regeneration phases (increased fuel consumption), has a limited temperature operation window and is again costly and prone to failure due to fuel contamination, in particular sulphur.
Even where EGR or throttling is used to control the intake air flow rate supplied to the engine to control the air/fuel ratio, the slow response time of the air flow compared to the rapid response time of the fuel supply leads to large variation in the air/fuel ratio. For example, every time there is a sudden change in torque request, there is a sudden change is the amount of fuel supplied to the combustion chamber, resulting in a sudden change in the air/fuel ratio. Every time the driver asks for less torque, the sudden reduction in the fuel supply will result in an immediate increase in lambda (i.e. a change to a lean mixture).
The present invention aims to provide an improved method of operating a compression ignition engine where greater control of the air/fuel ratio is achieved without compromising driveability or engine responsiveness.